This invention relates to systems for controlling movements of a workpiece along a path, and particularly to such systems where backlash is substantially reduced for avoiding uncontrolled movements of the workpiece.
This application incorporates by reference the teachings of application Ser. No. 09/427,392 entitled xe2x80x9cApparatus and Method For Fabricating Optic Fiber Preformsxe2x80x9d, filed Oct. 25, 1999, simultaneously with the filing of this application.
In numerous manufacturing processes, a carriage is moved back and forth along a path, an example being a lathe where a tool is moved back and forth along a path parallel to the axis of a rotating spindle. In some processes, it is essential that the rate of movement of the carriage be extremely precise and controlled. While it is generally possible to achieve highly controlled movements of the carriage in any direction, a problem generally exists at the ends of the carriage path where the carriage must stop and change direction. The problem arises from the fact that even in the most precisely machined apparatus, some degree of looseness of fit among parts of the system exists either directly from the manufacturing tolerances or from inevitable wear. Thus, when the carriage, moving in one direction, first stops and begins to move in the opposite direction, some inevitable lag, and accompanying uncontrolled movement of the carriage, occurs until parts in the drive train, pushing against one another in one direction, move through tiny gaps for re-engaging and pushing one another in the opposite direction.
Various means, such as constant friction members and spring loaded members are known for maintaining parts of a driving train in constantly stressed relationship for avoiding backlash. The present invention provides a novel arrangement which is quite simple and relatively inexpensive to implement.
First and second independent force generators (e.g., known a.c. or d.c. motors) provide controlled and variable amounts of force to a carriage to cause movement of the carriage in a first direction or in a second, opposite, direction (e.g., forward and reverse directions). At certain periods of time, the forces from first and second independent force generators are simultaneously applied to the carriage so as to oppose each other with the direction of movement of the carriage being determined by the larger of the two forces. The forces of the first and second force generators are always applied at the ends of travel of the carriage just prior to the carriage coming to a stop and reversing its direction of movement, e.g., from forward to reverse. Thus, at each end of travel, when one of the first and second force generators is exerting a force causing movement of the carriage in one direction, the variable force of the other one of the first and second force generators which, when large enough, causes reverse direction movement of the carriage, is already being applied against the carriage. Backlash is thus significantly reduced, if not avoided.
In one embodiment, both forces are applied continuously against the carriage, with the rate of movement being determined by the larger of the forces, and with the smaller force providing braking for greater precision of the driving of the carriage by the larger force.